1. Field of the Invention
The present invention relates to a band-pass filter and related frequency down converter, and more particularly, to a self-matching band-pass filter and related frequency down converter.
2. Description of the Prior Art
A satellite communication system capable of wideband and coverage is widespread use in many areas, such as probe, military, telecommunication network, data communication, mobile communication, etc. A ground user of the satellite communication system requires a device consisting of an antenna, a low-noise block down-converter (LNB), and a demodulator for receiving a satellite signal. After the satellite signal is received by the antenna, the satellite signal is down converted to an intermediate frequency (IF) signal via the LNB, and finally demodulated to a transmitted signal via the demodulator for outputting into a user device, such as a television.
Please refer to FIG. 1, which is a functional block diagram of an LNB 10 according to the prior art. The LNB 10 includes a low noise amplifier 100, a band-pass filter 102, a matching circuit 104, a mixer 106, and a local oscillator 108. A radio frequency (RF) signal VRF is received by an antenna, and enters the LNB 10. Then, the RF signal VRF is amplified via the low noise amplifiers 100, and is filtered image frequency signal out via the band-pass filter 102 to generate a filtered signal VFRF. Finally, the filtered signal VFRF is down converted to an IF band via the mixer 106 to output an IF signal VIF. The band-pass filter 102 is a microstrip band-pass filter, which can be implemented by the following filters: a hairpin band-pass filter, an end-coupled band-pass filter, and a parallel-coupled band-pass filter as shown in FIG. 2A-2C.
Please refer to FIG. 2A, which is a schematic diagram of a hairpin band-pass filter 20 according to the prior art. The hairpin band-pass filter 20 includes an input port PIA, an output port POA and resonators RNA—1-RNA—n. The input port PIA and the output port POA are respectively coupled to a front-stage circuit and a rear-stage circuit for receiving and outputting signals. Trace width W of each of the resonators RNA—1-RNa—n is the same, and total length of each of the resonators RNA—1-RNa—n is around half of a wavelength λ corresponding to a received signal. FIG. 2B is a schematic diagram of an end-coupled band-pass filter 22 according to the prior art. The end-coupled band-pass filter 22 includes an input port PIB, an output port POB and resonators RNB—1-RNB—n. The resonators RNB—1-RNB—n are parallel lines and trace width W of each of the resonators RNB—1-RNB—n is the same. In addition, length L of each of the resonators RNB—1RNB—n is λ/2. FIG. 2C is a schematic diagram of a parallel-coupled band-pass filter 24 according to the prior art. The parallel-coupled band-pass filter 24 includes an input port PIC, an output port POC and resonators RNC—1-RNC—n. The resonators RNC—1-RNC—n are parallel lines and trace width W of each of the resonators RNC—1-RNC—n is the same. In addition, length L of each of the resonators RNC—1-RNC—n is λ/2, and adjacent resonators overlap for a length of λ/4. An amount of resonators of the abovementioned band-pass filters is related to filtering performance of the band-pass filters.
In order to cooperate a characteristic of a coaxial cable of a measurement instrument, an input impedance to an output impedance of the band-pass filter 102 are usually set to 50Ω:50Ω. The mixer 106 is constructed by active elements, such as a field effect transistor or a bipolar junction transistor, whose input impedance is usually lower than the output impedance of the band-pass filter 102. The matching circuit 104 is used for controlling impedance match between the band-pass filter 102 and the mixer 106, so as to reduce the RF signal loss during transmission.
In the prior art, the rear-stage circuit elements are not particularly considered for the input and output impedance designs of the band-pass filter. Therefore, in the conventional LNB, the output port of the band-pass filter needs to be coupled to the matching circuit, and then performs impedance match with the rear-stage circuit. However, when the matching circuit exists, transmission line effect cannot be decreased to the lowest level and the loss of the RF signal outputted from the band-pass filter to the mixer cannot be improved effectively.